In fiber optic communication systems, transmitting optical subassemblies (TOSA) and receiving optical subassemblies (ROSA) are employed in order to transmit and receive optical signals over optical cables. For example, a ROSA or TOSA comprises a TO-can (TO: transistor outline) with a multi-pin sub-assembly package in order to house a laser or a photodiode. Widely used are 4-pin, 5-pin, or 6-pin TO-cans. The multiple pins of a TO-can are designed to receive an analog transmit signal in order to control the transmission of light by a laser or to deliver an analog receive signal in correspondence of light detected by a photodiode. A TOSA or ROSA comprises a TO-can and attached thereto a cap or flange, such that an optical cable can be precisely connected and a precise transition of light is established between the laser and the optical cable, respectively between the optical cable and the photodiode.
The multiple pins of a TOSA or a ROSA are electrically connected to a PCB (PCB: printed circuit board). In widely used applications, a TOSA is arranged beside a ROSA, in order to arrange for transmitting and receiving capabilities on the same PCB. The PCB comprises electronic driver circuits, which build an interface to a communication interface designed to be connected to a corresponding interface of a computer, for example. The PCB is arranged in a housing, such that an optical transceiver module is obtained, which has on one side an electrical interface for the connection to a electronic equipment and on the other side an optical interface for the connection to optical cables.
US 2010/0054754 discloses a 5-pin receiver optical sub-assembly (ROSA), wherein two pins are connected to a voltage source, two pins are output pins that output a differential output signal and one pin is a dual-purpose pin. A photodetector generates an electrical signal, which is converted with an optical data receiver circuit into a digital data stream that is transmitted out of the ROSA via the two output pins as a differential signal. External to the ROSA, a light level monitor circuit is connected to the dual-purpose pin. A 5-pin transmitter optical sub-assembly (TOSA) includes an optical data transmitter circuit with a differential buffer, a driver and a light emitting device. The differential buffer is configured to receive a differential data signal stream that is fed into TOSA via two input pins. It is converted to a single-ended data stream, which is coupled into the driver in order to drive the light emitting device, which may be a laser.
Currently available optical sub-assemblies are designed for a particular application. Hence, for each application a specially designed optical sub-assembly has to be developed and produced. In currently available optical sub-assemblies, various circuits like a TIA (TIA: transimpedance amplifier) or circuits for transforming between a RF signal (RF: radio frequency) and a binary data stream are arranged directly within the optical sub-assembly. However, when using currently available optical sub-assemblies in transceiver modules, the need for additional external circuits remains still high, particularly in order to monitor operation of the optical sub-assembly and generate required control signals.